1. Field of the Invention
This invention generally relates to digital wrapper format communications and, more particularly, to a system and method for generating useful error alarms for digital wrapper communications using forward error correction statistics.
2. Description of the Related Art
Digital signals are typically processed as a serial data stream and recovered using amplitude and timing parameters. Alternately stated, the amplitude of the digital signal must be monitored to detect the difference between a “1” bit and a “0” bit. In addition, a clock must be generated to differentiate bit positions. In transmission, a signal can undergo several stages of amplification, attenuation, and limiting. Further, the transmission medium may act to degrade the signal. To recover the signal at a receiver, the signal voltage swing is analyzed around a threshold voltage. Signals above the threshold can be considered a “1” bit, while signals below the threshold can be considered a “0” bit. If the threshold is set correctly, no bits are misinterpreted in the threshold analysis. However, if the threshold is set incorrectly, then errors will result as the signal is converted into digital information. Errors can be introduced by a variety of other means, besides thresholding. For example, the errors can be random in nature, or occur as the result of a faulty amplifier or leveler circuit. The errors can be introduced by the transmitter, the receiver, or can be introduced in the transmission media.
Some forms of digital communications include forward error correction (FEC) information included as overhead. Using the FEC information, degraded or misinterpreted bits can be recovered. Networks that are built to be compliant to the International Telecommunications Union ITU-T G.709 (G.709) specification have the requirement of monitoring the quality of the data received at each node in the network. For this reason, G.709 defines a byte in the overhead field for monitoring the bit error density, called Bit Interleaved Parity (BIP). The BIP byte in each frame is then collected over time to generate the signal fail (SF) and signal degrade (SD) alarms. The bit parity is calculated for each bit position of the byte. Thus, there are eight parity checks. The parity is calculated by adding all the bits from a particular bit position in every byte for an entire frame.
However, in G.709, the received BIP byte is calculated after errors have been corrected with the FEC. In order for the BIP in any individual frame to reflect any errors at all, the error density in one or more sub-rows of the frame must be higher than 8 bytes out of 255. This is because the standard FEC algorithm used in G.709 networks is the Reed-Solomon (255,239) algorithm which has the ability to correct all errors in a 255 byte codeword, if there are no more than 8 bytes out of each 255 bytes received in error. Thus, the Reed-Solomon (255,239) algorithm used in G.709 systems will correct up to 8 byte errors in each 255-byte block.
If more than 8 bytes in each 255-byte block are in error, the Reed-Solomon (255,239) algorithm is no longer able to correct the errors. At this error rate, the BIP itself is not of much use because it has a high probability of being incorrect. Additionally, an error rate of more than 8 bytes out of 255 bytes corresponds to an error rate of greater than 3*10−2, which is typically much higher than can be tolerated in the networks that are built to be compliant to G.709. Thus, using the BIP method for generating SD and SF means that neither the SD nor the SF will get set until the error density is much higher than can be tolerated in the network.
At a BER of 10−12, a fairly typical error rate, the FEC corrects all the errors, and the BIP indicates no errors. However, as the number of errors approach the point at which the FEC can no longer correct all the errors, the BIP will have long since been useless as an indicator of error information.
Many carriers or network service providers like to specify their system's performance with a guaranteed BER for a particular bandwidth (data rate). If fact, services are often sold using such specifications. This information cannot be provided from the BIP-8 bytes. Typically, received frames must be parallel processed with additional equipment to determine BER data.
It would be advantageous if there were a more accurate method of generating SD and SF alarms in a G.709 compliant system.
It would be advantageous if error information could be generated in response to the FEC information, instead of BIP.